1. Field of the Invention
The present invention relates to a method of tuning a frequency synthesizer system with multi-band voltage controlled oscillators (VCOs), and more specifically to tuning a wireless frequency synthesizer system with multi-band VCOs having very limited initial frequency lock times.
2. Description of the Prior Art
In a frequency synthesizer system with multi-band voltage controlled oscillators (VCOs), it is required to tune the multi-band VCOs first, before tracking the desired frequency f0. This is done to ensure that the selected VCO covers the desired frequency f0 and that the synthesizer can be locked at f0.
U.S. Pat. No. 6,731,712 issued to Lindstrom et al. discloses one such system where the frequency spectrum is divided into a plurality of ranges with each VCO covering one range. When tuning the system, the upper and lower boundaries of each VCO frequency range is found by recursive scanning, employing smaller and smaller step sizes in both directions away from a center frequency for that VCO. When a specific frequency is later desired, the VCO whose range contains that specific frequency is uniquely selected for tracking and locking.
Please refer to FIG. 1 as an example prior art tuning procedure. In Step 150, a desired frequency f0 is obtained. This is followed in Step 155 by a tuning algorithm similar to that described above. Once the VCOs are tuned, the synthesizer tracks the frequency f0 in Step 170 and the frequency f0 is locked in Step 180 where it remains locked until the frequency changes in Step 190 and the entire process is repeated.
Tuning methods such as this are suitable for most wireless systems because most wireless systems have a much relaxed timing requirement when the channel is changed. However, some wireless systems require the synthesizer to be locked at the desired frequency as quickly as several 10 microseconds and there is no extra time to adopt the prior art VCO tuning procedure.